Praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
You (Questioner) and your husband aren`t obliged to help with the financial expenses of his brother`s marriage. Rather, it is desirable for both of you to help with whatever amount you can afford as charity, for Allah the Almighty burdens not a soul beyond its scope.
Undoubtedly, paying off urgent debts takes precedence over giving charity. The most important thing, here, is that you get out of this situation in the best way possible. This is to avoid discord and preserve familial ties. And Allah the Almighty Knows Best.